1. Field of the Invention
The present invention relates to a technique for stabilizing and minimizing a cogging torque which is generated in a brushless DC motor provided with nine slots and eight poles.
2. Description of the Related Art
The order of cogging toque is generally calculated by applying the least common multiple of numbers of slots and magnetic poles. When the degree is high, the cogging torque is small. As a technique relating to cogging torque, a technique disclosed in Japanese Examined Patent Application, Publication No. 58-42708, is known. In the disclosed structure, the cogging torque is reduced by auxiliary slots 15 formed on an auxiliary salient pole 14 and prime salient poles 12a and 12b. Furthermore, as a technique for avoiding vibration, Japanese Patent Application, First Publication No. 8-214525, discloses a technique applied to a permanent-magnet-type of a salient pole DC motor in which the net radial force between a rotor and a stator is reduced by balancing of the radial reluctance forces therebetween. In the technique, balancing of the radial reluctance is obtained by slotting at a pole face of a pole tip of the salient pole of the motor.